Temporary workaround for #6437: always allow blind writes (but log the issue)

For #6437, allow a file write to succeed even if we think the timestamp is newer. In general, we get reliable notifications from file watchers when files change. However, in rare cases on Mac we appear to see a change in modtime even when the contents of files haven't changed, and that isn't accompanied by a file watcher notification. For now, we're simply disabling the check.

/cc @peterflynn